The order protection rule aims to ensure that investors get the best price when their order is filled by eliminating the possibility of orders being sold (executed at the worst price).
It requires stocks to be traded on exchanges that show the best quotes and requires trading centers to establish and enforce written rules and procedures to ensure this.
The order protection rule is a provision of the National Market System Regulations (NMS), a set of rules adopted by the SEC in 2005, and is also referred to as the “trade through” rule.